FarHorizon

If your frame is steel or thin wall aluminum and your seat post diameter is 27.2mm, then a 31.8 clamp will fit. If your frame is aluminum and the seat tube walls are thicker, then a 31.8 clamp will NOT fit. You need the 34.9mm size. The only ways to tell for sure are to either measure the seat tube at the clamp location OR to see what size the factory clamp was (some, but not all, have the size stamped on them).

Do NOT try to pry open a 31.8 clamp to fit a 34.9 seat tube. The pinch-bolt either won't be long enough and/or the holes will no longer be parallel.

I speak from experience after ordering not one but two 31.8mm clamps that won't fit my aluminum frame!

I reopen this 2006 thread because a Google search directed me here.

PS: For my bike, a 34.9mm clamp won't fit either! My seat tube diameter is 33.6mm! How ******** is that? I guess I'll need to use a 34.9 with shims? The only clamp offered by the manufacturer is a QR type, and I want a bolt-on to prevent snatch-and-run theft of my expensive Brooks saddle. If I can find a hemispherical metal washer (to fill the QR slot on the factory clamp), then I can modify the thing into a bolt-on... Hardware store, here I come!